This invention relates to fuel cell stacks and, in particular, to gas cooled fuel cell stacks.
In general, gas cooled fuel cell stacks are provided with cooling assemblies which are distributed throughout the stack length. Successive cooling assemblies typically sandwich a plurality of fuel cells which are cooled by the assemblies. A fuel cell stack of this type is disclosed in U.S. Pat. No. 4,192,906.
It sometimes happens in large stacks of the aforesaid type that one or more fuel cells become defective or inoperative and therefore have to be removed from the fuel cell circuit. Copending U.S. application Ser. No. 338,354, filed Jan. 11, 1982, and, assigned to the same assignee hereof, discloses a fuel cell stack in which removal of a defective cell is carried out by physically replacing a fuel cell sub-assembly which contains the defective cell. As taught in this application, replacement of the sub-assembly is greatly facilitated by forming each end of the sub-assembly, readily disconnectable component of a cooling assembly.
While the aforementioned practice of replacing subassemblies containing defective cells is highly advantageous, it does entail the added expense of providing replacement subassemblies. Also, while the replacement procedure is comparitively simple, it takes time which results in downtime for the stack.
In stacks of the above type employing a large number of cells such as might be used by public utilities, the occurrence of a few defective cells is not of significance to the stack output. Thus, physical replacement of the cells is not required from a performance standpoint, but only to realize a closed electrical circuit. Furthermore, in large stacks of this type, unscheduled downtime has to be kept at an absolute minimum. As a result, downtime to replace an entire sub-assembly due to cell failure is undesirable, especially when it is not required to achieve suitable stack output.
One way to alleviate this problem would be to short circuit the defective cell or cells. To be useable, however, a short circuit arrangement would have to be mechanically and electrically sturdy, since it would be subject to physical abuse and would be required to operate for 40,000 hours or more. Furthermore, it would have to exhibit a minimum of electrical resistance drop and interfacial resistance. Finally, and quite importantly, it would have to distribute the current to the cells immediately adjacent the defective cell as uniformly as possible so that such adjacent cells and the remaining cells maintain acceptable performance.
To provide a short circuit arrangement having the aforementioned characteristics between cells would be extremely difficult and time consuming, if not impossible. This is due to the fragile nature of the individual cell components which are extremely thin. As a result, attempts at shorting between cells could lead to damaging of the cells as well as to a resultant short whose mechanical strength and resistance are unsuitable.
It is therefore an object of the present invention to provide a practice for shorting a fuel cell stack which does not suffer from the aforesaid disadvantages.
It is a further object of the present invention to provide a shorting connection for a fuel cell stack which exhibits high mechanical and electrical strength, low resistance drop and interfacial contact resistance and uniform current distribution.